Neoplastic cells from patients infected with HTLV generally express receptors for T-cell growth factor (TCGF) (Interleukin-II) and do not require prior activation with antigens or lectins to undergo TCGF-induced proliferation. Furthermore, neoplastic T-cell lines originating from such patients may constitutively produce TCGF, TCGF receptors, and HTLV virions. HTLV is transmissable from cell to cell, and the infection of human T cells in vitro is associated with expression of TCGF receptors, which can be identified by the monoclonal antibody termed anti-Tac. In our experience to date, T-cell populations that produce HTLV also express epitopes found on TCGF receptors without exception. Recognition of an association between HTLV virions and the Tac antigen would have clinical and theoretical implications. We present evidence that during the replication of release of HTLV the virion becomes preferentially associated with the Tac antigen. As previously reported, the 55,000-dalton glycoprotein detected by the monoclonal antibody anti-Tac is a normal T-cell activation marker that is identical or very closely linked to the receptor for TCGF. We have been able to detect the Tac antigen in preparations of the neoplastic T-cell line HUT-102-B2 derived from a T-cell lymphoma patient C.R. and in pelleted virus (HTLVCR), which is released by this line, using anti-Tac containing ascites and an enzyme-linked immunosorbent assays (ELISA). The anti-Tac antibody readily bound to the purified (double-banded) HTLV preparation over a range of dilutions, while equivalent concentrations of control murine ascites did not. A similar pattern of binding was observed when a preparation of HUT-102-B2 cells was assayed. By contrast, the anti-Tac antibody did not bind to an Epstein-Barr virus transformed B-cell line (CR-B) derived from the same patient, or to the T-cell line HUT-78, which did not release HTLV.